A Study Of Decreasing Intermediate Bulk DC-LINK Capacitors For Three-Phase Cascaded PET

Due to the increasing of distributed power supply,the diversification of user’s demand at the same time,the traditional urban ac power grid faces more and more technical challenge.In the Future,the urban distribution network must be a new kind of power distribution network which is necessarily supported by traditional urban distribution network,and can realize ac power,dc power and ac/dc hybrid power distribution network interconnection of different voltage grade.Power Electronics Transformers(PETs)with direct connection between Medium-Voltage(MV)grid and Low-Voltage(LV)DC distribution systems are a promising technology in the future,which is a good choice due to its modularity and availability for different levels and nature of voltage in the two independent ac/dc power grid,and which achieve power transmission,stable and coordinated control functions.However,the PET exist many problems,such as the influence of the power fluctuation of the three-phase cascaded PET leads the intermediate bulk DC-link capacitors and low power density.Therefore,it is the main content how easily and effectively solves them.In this paper the Sinusoidal Power Transfer(SPT)concept is proposed for three-phase cascaded PETs which own LLC resonant converter,since it can enable a considerable reduction for the intermediate DC-link capacitance while transfering per-phase Double-Line-Frequency(DLF)power fluctuation to the common low-voltage DC side and maintaining the constant DC power supply.In addition,the LLC resonant converters working at quasi-resonant mode are skillfully adopted to realize SPT and simplify the PET control system structure based on single-stage control system concept.The SPT limitation for PET is having narrow power factor control capability,but SPT concept is a new trend for unidirectional PET to reduce the volume and the cost.According to the three-phase PET topology and SPT concept,the characteristics of three-phase PET are analyzed.Finally,a hardware prototype of three-phase PET based on LLC resonant converter has been designed and tested.The contrasts with experimental results and characteristic analysis have verified the validity and feasibility of the SPT concept using LLC resonant converters for three-phase cascaded PETs.